1. Field of the Invention
The present invention relates to an improved method for preparing anionic polymers utilizing water-in-oil emulsion polymerization.
2. Description of the Prior Art
Water-in-oil emulsion polymerization was suggested by Vanderhoff in U.S. Pat. No. 3,284,393. The Vanderhoff technique proposed polymerizing water-soluble monomers utilizing a water-in-oil emulsion polymerization system. The process of Vanderhoff utilized a water-in-oil emulsifying agent to emulsify the water in the oil and conventional polymerizing initiators. The resulting polymer emulsion, a polymer latex, could then be broken to recover the polymer or utilized as is. The Vanderhoff polymerization technique provides a good technique for producing nonionic polymers, however, attempts to utilize the Vanderhoff system to prepare anionic polymers has proven to be less than successful. When preparing anionic polymers, rather than obtaining a polymer latex, one obtains a polymer mixture composed of polymer latex and gritty polymer particles, that is nonemulsified polymers.
Water soluble polymers produced by the Vanderhoff technique are used as flocculants and rheology modifiers for aqueous systems. Anderson in U.S. Pat. Nos. Re. 28,474 and Re. 28,576 discloses a technique for rapidly dissolving the water soluble polymers prepared by the Vanderhoff technique in aqueous medium. Briefly, the Anderson technique involves forming a water-in-oil emulsion of the polymer and then inverting this emulsion into the aqueous medium. According to Anderson, this technique provides for rapid dissolution of the polymer into the water. The Anderson technique suffers from the distinct drawback that one first must prepare the water-in-oil emulsion. This emulsion can be prepared by emulsifying solid polymer or alternatively by conducting the polymerization utilizing the Vanderhoff technique and then inverting the resulting product in an aqueous medium. Obviously, utilizing the product of the Vanderhoff polymerization technique, the polymer latex, would be far more efficient than forming a water-in-oil emulsion from solid polymer. With nonionic polymers this appears to be possible, however, with anionic polymers the Vanderhoff polymerization technique produces a gritty polymer product which is not suitable for commercial use and it is necessary to first remove the grit from the polymer latex. Furthermore, emulsions whether prepared by the Vanderhoff technique or the Anderson technique have proven to have unsatisfactory short shelf lives.
Kane in U.S. Pat. No. 3,997,492 discloses a polymer emulsion system suitable for dissolving a polymer in water comprising finely divided polymer, aliphatic hydrocarbon liquid and a combination of nonionic surfactants. The choice of the surfactants is dictated by the properties of the aliphatic hydrocarbon liquid, in particular the London coefficient unit and other factors. When the Kane technique is applied to anionic polymers an unsatisfactory emulsion results.
Accordingly, a need continues to exist for a technique providing stable water-in-oil emulsions of anionic polymers.